Summary: Write alt text that communicates an image's purpose without repeating page content. Focus on meaning rather than visual description.
Alt text (or alternative text) is metadata intended to help screen-reader users understand what an image is depicting. Most guidance for writing alt text describes how to make individual web-based images accessible and understandable for all users. This is more complicated than it sounds because users encounter alt text while trying to accomplish more complex tasks that generally involve interacting with many elements on the page. Images are just part of the picture.
What Users Need from Alt Text
Assistive-technology users obviously want individual page elements to be accessible, but (just like anyone else) what they want most is an easy path to accomplish basic tasks . Accessibility is a means to the end of whatever the user needs to accomplish.
Even if every image has “perfect” alt text, our job isn’t done until assistive-technology users can do what they came to do. That is the argument we’ve been making for nearly 20 years : the focus must be usability — not just accessibility.